The present invention relates to a negative electrode for a lithium secondary battery that includes an aluminum alloy as a negative electrode active material. More specifically, the present invention relates to an improved negative electrode active material which provides a negative electrode with which it is possible to prepare a lithium secondary battery having excellent charge-discharge cycle characteristics.
When a lithium metal plate is used for a negative electrode for a lithium secondary battery, active dendritic lithium is deposited out during charge, and the deposited lithium reacts with the electrolyte to decrease the negative electrode capacity and causes an internal short-circuit of the battery when it grows by repeated charging and discharging. If a lithium-aluminum alloy sheet that is electrochemically prepared from lithium and crystalline aluminum is used instead of a lithium metal sheet, reaction of lithium with an electrolyte and formation and growth of dendritic lithium by repeated charge-discharge cycles are inhibited, and charge-discharge cycle characteristics are improved. However, electrochemical reaction (alloying reaction) rate of lithium and crystalline aluminum is low, and a dramatic improvement of charge-discharge cycle characteristics cannot be expected.
It has been proposed that a lithium-aluminum alloy sheet that is prepared electrochemically from lithium and amorphous aluminum be used for a negative electrode for a lithium secondary battery instead of the above-explained lithium aluminum alloy sheet (Japanese Patent Laid-open Publication No. 63-13267). According to this publication, the reaction rate of the electrochemical reaction between lithium and amorphous aluminum is higher than that between lithium and crystalline aluminum, and charge-discharge cycle characteristics are improved as well.
However, the inventors of the present invention found that inactive Li2O is deposited out on the surface of a negative electrode because the lithium aluminum alloy sheet has a small contact (reaction) area between the alloy sheet and an electrolyte. Therefore, the obtained charge-discharge cycle characteristics are not satisfactory.
Objects of the present invention are to provide a negative electrode for a lithium secondary battery that has excellent charge-discharge cycle characteristics and to provide a lithium secondary battery which includes the negative electrode.
The present invention provides a negative electrode for a lithium secondary battery comprising aluminum alloy powder as an active material,
wherein the alloy is substantially amorphous and is represented by the formula AlxSiyMz;
where M is at least one transition metal selected from the group consisting of Ni, Co, Cu, Fe, Cr and Mn;
x, y and z are 40xe2x89xa6xxe2x89xa680; 10xe2x89xa6yxe2x89xa650 and 1xe2x89xa6zxe2x89xa620, respectively, and x+y+z=100; and
average particle diameter of the alloy is not greater than 50 xcexcm.